Spotlight

Researchers at the School of Physics and Astronomy are trying to solve a thirty year old mystery: what causes high-temperature superconductivity? Guichuan Yu is a postdoc in a lab that is entirely focused on the problem. Yu has been with Martin Greven’s superconductivity laboratory since he was a graduate student and has spent the past ten years refining the technique of creating one class of materials, Mercury Barium Copper Oxides. As a postdoc he is now in charge of growing the single crystal samples the group uses in their experiments.

These materials are arguably the most ideal for studying high temperature superconductivity. “One member of this particular material family has the highest transition temperature known so far,” Yu says. They also have a relatively simple crystal structure which makes them ideal for study.

Superconductivity was first discovered in 1911, when physicists first liquefied helium. This allowed them to cool materials to temperatures close to absolute zero. They found that ordinary metals like mercury lost all resistivity when cooled to these temperatures. It wasn’t until the 1950s that the quantum phenomenon that drove superconductivity were fully understood. In the 1980s it was discovered that certain ceramic materials, previously thought to be insulators, were superconducting at temperatures about half-way between absolute zero and room temperature. “The transition temperature is way too high for the classical theory to work,” Yu says. “In a classical superconductor there are crystal lattice vibrations that attract the electrons together to form pairs and those pairs of electrons move without resistance. In high temperature superconductors, people have never confirmed what exactly is causing those electrons to pair up.”

Thirty years later with more than 200,000 research papers published in this field, physicists still don’t understand what causes these materials to be superconducting. The technology has a host of potential applications in the power industry as well in industries that require extremely powerful magnets, such as Magnetic Resonance Imaging (MRI). High temperature superconductors still require being cooled with liquid nitrogen to work which makes them less desirable as devices. In order to create a superconductor that works at room temperature, scientists need to figure out what causes the phenomenon in the first place.

Part of the Yu’s job is to create superconducting materials that are doped with extra charges at different levels. This way the material can be tuned to show various kinds of exotic properties. To try to understand these properties the group takes their samples to x-ray scattering and neutron scattering facilities. “We shoot the neutron or x-ray beam into the sample and look at how it scatters the beam. It gives us a picture of the sample properties.” Neutron scattering is used to understand the magnetic properties of the materials and X-ray scattering helps them understand how the charge organizes in real space.

The group has come up with some exciting results. A paper published last year showed charge waves in their materials. A recent paper showed that certain magnetic waves which had frequently been seen with high temperature superconductors, was not present. This would seem to suggest that charge waves may play a part in driving superconductivity but magnetic waves probably do not.